It is more accurate to say that Apple defaults to IPv6 and removed it from the user to change to a IPv4 connection. If you need to connect to say a sercurity video feed that uses IPv4 you will need a android device since it allows you to create a new APN to force a IPv4 connection. You can also use a VPN.
Apple says "You can only edit or view your APN if your carrier allows it", and it appears from talk on Apple forums that T-Mobile's carrier settings remove the ability to change the APN. However, that's not the core of my question.
We can successfully connect to IPv6 addresses on TMobile with Mobile Safari if and only if the NAT64-mapped IPv4 address is used. Hence, as a workaround it would be useful to know what NAT64 prefixes might be in use so that I can populate a list using those.